What exactly does a shell script do and what will it be used for?

Shell follows the classic UNIX philosophy: break down complex problems into simple small problems, and then combine the functions of each part to solve complex problems.

For example, I can use a shell script to monitor the overall performance of the server. When the CPU load exceeds my preset warning line, the threshold of disk space exceeds my preset standard, and the server is down, all these can be sent to me automatically by writing a script or sending me a text message through the shell. Simplify tedious daily work steps, data backup and log collection. Too much, too much. ..